1. Field of the Invention
The present invention relates to a driver IC having a group of drive elements that individually drive external elements such as light-emitting diodes for an exposing unit for an electrophotographic printer, heat-generating resistors for a thermal printer, display elements for a display.
2. Description of the Related Art
One conventional driver IC is an LED driver IC that drives a plurality of light-emitting diodes (LED) of an LED array. The LED driver ICs and LED arrays are combined to form an LED head for use in the electrophotographic printer.
FIG. 17 illustrates a layout of the conventional driver IC fabricated using the technique disclosed in Japanese Patent Preliminary Publication (KOKAI) No. 6-297765. FIG. 17 also shows a see-through view of a layout of an LED array 100 for convenience of explanation. The LED array 100 includes 192 LEDs. Each chip of the driver IC 400 drives 192 LEDs.
The driver IC includes a shift register 201, a set of latches 202, a set of pre-buffers 204, a set of drive transistors 205, a set of input electrode pads 210, a set of drive electrode pads 220, a supply electrode 230, and supply electrode pads 431. The shift register 201 includes flip-flops FF1-FF192. The set of latches 202 includes latches LT1-LT192. The set of pre-buffers 204 includes pre-buffers G1-G192. The set of drive transistors 205 includes drive transistor Tr1-Tr192. The set of drive electrode pads 220 includes drive electrode pads DO1-DO192. The reference "A" denotes the region between even-numbered drive electrode pads (DO2, DO4, . . . , DO192) and the supply electrode 230.
The driver IC 400 is in the shape of a rectangle chip having long sides and short sides. The set of the drive electrode pads 220 extends along one of the long sides of the driver IC 400 and includes drive electrode pads DO1-DO192 arranged in two rows with one row staggered with respect to the other. The drive electrode pads DO1-DO192 are used to drive 192 LEDs of the LED array 100, each pad being connected to a corresponding LED. The sets of input electrode pads 210 are aligned along the other long side of the driver IC 400 and receive signals from outside.
The shift register 201, set of latches 202, set of pre-buffers 204, set of drive transistors 205 are aligned in this order from the input electrode pads 210 side to the drive electrode pads 220 side. The flip-flops FF1-FF192, latches LT1-LT192, pre-buffers G1-G192, and drive transistors Tr1-Tr192 are all aligned in the direction of the long side of the driver IC 400 at the same intervals as the drive electrode pads 220.
The supply electrode 230 is an aluminum electrode having a width W, and is formed between the set of drive transistors 205 and the set of pre-buffers 204. The supply electrode 230 extends from one short side of the driver IC 400 to the other in the direction of the long side of the driver IC 400.
Provided on the supply electrode 230 are two electrode pads 431 via which drive supply VDDH is supplied from outside. The sources of the drive transistors Tr1-Tr192 are connected to corresponding nearby supply electrodes 230.
The two supply electrode pads 431 are at locations corresponding to the drive transistors T64 and Tr132, respectively.
The LED array 100 is in the shape of a rectangle chip having long sides and short sides, and has a set of anode electrode pad 120 that includes anode electrode pads LI1-LI192. The set of the anode electrode pads 120 extends along one of the long sides of the chip of LED array 100. The anode electrode pads LI1-LI192 are arranged in two rows with one row staggered with respect to the other. The anode electrode pads LI1-LI192 are connected to the corresponding drive electrode pads DO1-DO192 using a later described ACF. An i-th anode electrode pad LIi is at a location corresponding to an i-th drive electrode pad DOi. The input electrode pads 210 are aligned along the other long side of the LED array 100.
FIG. 18 is a lateral cross-sectional view of a chip module having the LED array 100 and prior art driver IC 400, taken along lines C-C' of FIG. 17.
Referring to FIG. 18, there are provided light emitting elements 101, individual anode electrodes (Al electrode) 102, and anode electrode pads (Al electrode pad) 103 on the surface of the LED array 100. Each combination of light emitting element 101, individual anode electrode 102, and anode electrode pad 103 forms one of a plurality of LEDs. Each anode electrode pad 103 is connected to a corresponding anode electrode pad (LI1-LI192) of FIG. 17. There is provided a common cathode electrode 104 on the opposite surface of the LED array 100.
The LED array 100 has LEDs of an end-surface emission type in which light is emitted in a direction parallel to the surface of the pn junction of the light emitting element 101. The anode electrode pad 103 is formed in integrally continuous with the anode electrode 102, and is connected to one end of a corresponding drive electrode pad 221 of the driver IC 400. The other end of the anode electrode 102 is connected to the p-type region of the light-emitting element 101. The common cathode electrode 104 is connected to the n-type region of all the light emitting elements 101 (i.e., n-type substrate of the LED array).
The driver IC 400 has the input electrode pad 211 that is a part of the set of input electrode pad 210 (FIG. 17), the drive electrode pad 221 that is a part of the set of drive electrode pads 220 (FIG. 17), and the supply electrode pad 431.
The input electrode pad 211 includes an Al-electrode pad 211a and an Au (gold)-bump 211b formed on the Al electrode pad 211a. The drive electrode pad 221 is the i-th drive electrode pad DOi of FIG. 17 and includes an Al-electrode pad 221a and an Au (gold)-bump 221b formed on the Al electrode pad 221a.
The supply electrode pad 431 includes an Al-electrode pad 431a and an Au-bump 431b. The Al-electrode pad 431a is on an area of the supply electrode 230 near the drive transistor Tr132 (i.e., near drive electrode pad DO132).
The set of drive electrode pads 220 of the driver IC 400 is electrically connected with the LED array 100 through an anisotropic conductive film (referred to ACF), thereby forming a chip module. Dispersing electrically conductive gold-plated particles 303b in a thermosetting ACF resin 303a forms an ACF 303.
The driver IC 400 and LED array 100 are connected in an ACF connection process as follows:
The LED array is placed on the driver IC with the ACF sandwiched between the anode electrode pads 103 and the drive electrode pads 221. The LED array is mounted to the driver IC such that the anode electrode pads 103 of the LED array 100 are aligned with the corresponding drive electrode pads 221 of the driver IC 400. Then, the assembly of the driver IC 400 and LED array 100 is heated so that the ACF resin 303a is softened. Then, the LED array 100 is pressed against the driver IC 400 and the assembly is again heated at an elevated temperature. Thereafter, the assembly is cooled so that the ACF resin is set. In this manner, the both chips are mechanically bonded to each other and the gaps between the two are sealed. The conductive particles 303b sandwiched between the anode electrode pads 103 and Au-bumps 221b allows electrical connection between anode electrode pads 103 and corresponding drive electrode pads 221 without using wires.
The chip module is bonded to a printed circuit in the die-bonding process. During the die-bonding process, the input electrode pads 211 and the supply electrode pads 431 of the driver IC 400 are connected to corresponding signal output electrode pads and supply electrode pads on the printed circuit board through bonding wires (gold wires) 302a and 302b, respectively. The common cathode electrode 104 of the LED array 100 is connected to a ground electrode pad on the printed circuit board through a later described bonding wire 302c (FIG. 13).
However, as shown in FIG. 17, when all of the LEDs of the LED array 100 are driven simultaneously, drive currents supplied through the drive transistors Tr1-Tr192 to the LD1-LD192 are different from LED to LED (i. e., from dot to dot). There are two reasons for the variations in drive current: the first is the potential on the supply electrode 230 is not uniformly distributed along the supply electrode 230 and the second is the increases in the connection resistance between the drive electrode pads 221 of the driver IC 400 and the anode electrode pads 103.
When all the LEDs (all dots) of the LED array 100 are driven simultaneously, the drive current is supplied to the drive transistors Tr1-Tr192 through the resistance of wires and wire-bonded portions so that each LEDi receives the drive current through a corresponding drive transistor Tri. A problem with the prior art driver IC 400 is voltage drops resulting from not only the resistance in the bonding wires and voltage drops but also resistance between adjacent nodes of the supply electrode 230. The variations in drive current due to the voltage drops between adjacent nodes of the supply electrodes 230 becomes large with increasing number of LEDs to be driven.
FIG. 19 illustrates the distribution of potential along the supply electrode 230 when all the LEDs are energized simultaneously. FIG. 19 plots dot numbers as the abscissa and potentials as the ordinate.
The supply electrodes 431 of the driver IC 400, through which drive supply VDDH is supplied from an external source, are provided at nodes S64 and S132. The potential on the supply electrode 230 is a maximum at nodes S64 and S132, and decreases with increasing distance from the nodes S64 and S132 due to voltage drops across residual resistance between adjacent nodes of the supply electrodes 230. The potential is a minimum at nodes S1 and S192. Correspondingly, the current density is a maximum near the nodes S64 and S132 and decreases with increasing distance from the nodes S64 and S132.
Thus, if the resistances between adjacent nodes of the supply electrodes 230 can be decreased, the variations in drive current between dots can be decreased. However, if the width W (FIG. 17) of the supply electrode 230 is made wider in an attempt to decrease the wire resistance, then the driver IC will be large in size.
FIG. 20 illustrates the ACF 303 when it is crushed between the driver IC 400 and LED array 100.
As shown in FIG. 20, the ACF 303 may cover only a part of the drive pads DO1, DO191, and DO192 which are located at endmost of the two rows of drive electrode pads. Therefore, the number of conductive particles sandwiched between the Au bumps 221 of the drive electrode pad 220 and corresponding anode electrodes pads 103 is less than when the ACF 303 completely covers the drive pads DO1, DO191, and DO192. This causes the resistance between the drive electrode pads DO1, DO191, DO192 and the corresponding anode electrode pads 103 to increase, thereby reducing drive currents supplied to LD1, LD191, LD192 to result in large variations in drive current between dots.
The non-uniform potential along the supply electrode 230 and the variations in drive current are sources of deteriorated print quality of electrophotographic printers.
The worst case is that the electrical connections between some drive electrode pads and corresponding anode electrode pads may become completely open. Such poor electrical connection reduces the yield of chip module formed of the driver IC 400 and LED array 100.
The prior art driver IC 400 requires an unused area where no electrical elements are fabricated, the unused area being in a region extending over a part of the region A between the rows of the set of drive electrode pads 220 and supply electrode 230. This unused area makes the chip size of the driver IC 400 large. The unused area is provided for the following reason.
During the ACF connection process described with reference to FIG. 18, the softened ACF 303 spreads toward the supply electrode pads 431. A certain thickness of the ACF before heating is required in order that a sufficient number of conductive particles 303b are present between the anode electrode pads 103 of the LED array 100 and the Au bumps 221b of the driver IC 400 for good electrical connection. Some excess amount of ACF 303 should be allowed to spread toward the supply electrode pads 431.
If the ACF 303 spreads too far, it may cover the Au bump 431b, being an obstacle to bonding the wire to the Au bump 431b. The region A serves to be a buffer area to accommodate excess melted ACF.